1. Field of the Invention
The present invention relates to a method for molding a resin-product for forming a hole in the bottom portion of a shaft bore extending along a central axis of the resin-product to a predetermined depth in a resin-product. More particularly, the invention relates to a method for molding a resin-product which is able to prevent formation of a weld mark or line on the inner circumferential surface of the shaft bore of the resin-product during the molding and is also able to form, in the bottom of the shaft bore simultaneously with the molding of the resin-product, a hole operable as a relief port of air toward the exterior of the shaft bore.
2. Description of the Related Art
In a conventional resin-product, a shaft bore to be press-fit with a shaft member such as a coupling pin and a rotary shaft along the central axis of the resin-product is often formed as a through-bore. In this case, for example, as shown in FIG. 11, in a cavity 32 formed in a molding die 31 for forming a shape of a resin-product, a center pin 33 having a diameter approximately equal to that of a shaft member stands upright so as to erect in the cavity 32. A side gate 34 is provided on the side surface of the cavity 32 so that molten resin 35 is injected into the cavity 32 from the side, which is orthogonal to the center axis of the center pin 33.
In another molding die, a thin gating for injection (it will be hereinafter referred to as a pin gate) as indicated by reference numeral 37 in FIG. 11 is provided extending in parallel with the center axis of the center pin 33 on the top end of the cavity 32, and the molten resin 35 is injected from the pin gate 37 into a space between the inside surface of the cavity 32 and the outside surface of the center pin 33.
In FIG. 11, the molten resin 35 injected from the side gate 34 collides against the side surface of the center pin 33 as shown in FIG. 12, and branches into two flows of molten resin, and those branched molten resin flow along the center pin 33 as indicated by arrows. The leading ends of the flows of the branched molten resin 35, indicated by arrow heads, merge at a point on the side surface of the center pin 33 which is opposite to the side gate 34. The leading ends of the flows of the molten resin 35 have been cooled and at a low temperature as they flow. Therefore, the molten resin 35 having flowed in two directions and merged do not mix, so that a weld mark or line 36 is formed at the point where those two molten resin flows have met each other to join together. Then, in FIG. 11, the upper member of the molding die 31 is opened, and a sleeve pin 38 disposed around the center pin 33 is pushed up. As a result, a resin-product having a through-bore extending along the center axis thereof is extruded out of the molding die.
Also in the case where the molten resin 35 is injected from the pin gate 37 in place of the side gate 34 in FIG. 11, the molten resin 35 flows along the center pin 33 in opposite directions, in a similar state as shown in FIG. 12, meet on the side surface of the center pin 33 at the point which is opposite to the pin gate 37, and a weld mark 36 is formed at the point where those two molten resin flows meet each other.
In the case where a shaft member is inserted into the shaft bore of the resin-product having the weld mark 36 formed on the inner circumferential surface of the shaft bore, strength of the resin-product is sometimes reduced at the portion where the weld mark 36 is formed. In an extreme case, the resin-product will crack at the portion of the weld mark 36. In the case where a rotary body is manufactured with the resin-product, the portion of the weld mark 36 may provide a stepped part. The rotary body loses its weight balance and its stability of rotation.
One way to prevent the formation of such a weld mark 36 is that as shown in FIG. 13, a top end face 33a of the center pin 33 is located in front of a top end 32a of the cavity 32 to thereby provide a predetermined distance from each other, and the pin gate 37 is provided on the top end 32a of the cavity 32 on the extension line of the center axis of the center pin 33. With such a structure, as shown in FIG. 14, the molten resin 35 injected from the pin gate 37 collides against the top end face 33a of the center pin 33 and radially spreads as indicated by arrows. Then, as shown in FIG. 13, the molten resin uniformly flows around the center pin 33 and toward a rear end part 33b of the center pin 33 within the cavity 32. Accordingly, in this case, the arrow-headed leading ends of the flows of the molten resin 35 do not merge with each other, and thus no mark of merging like the above-mentioned weld mark 36 is formed. Subsequently, in FIG. 13, the upper member of the molding die 31 is opened, and the sleeve pin 38 disposed around the center pin 33 is thrust up, whereby a resin-product having an axially extending blind bore having a predetermined depth formed along the center axis thereof is extruded from the molding die.
However, this approach has the following problem. The shaft bore that is formed in the resin-product in order to be press-fit with the shaft member takes a shape of a bottomed blind bore. When the shaft member is press fit into the bottomed blind bore, the air confined in the bottomed blind bore cannot escape from there and is in turn compressed. As a result, the press-fitting of the shaft member is hindered and cannot be completed.
To cope with the problem, there has been a proposal that an axially lengthened bottomed bore having a predetermined depth is produced in the resin-product to allow the shaft member to be completely press fit in the bottomed bore while leaving a cavity portion between the shaft end and the bottom of the bottomed bore and, in addition, an opening or aperture is formed in the bottom of the axially lengthened bottomed bore (Japanese Unexamined Patent Publication No. 2002-268513). With such a structure, when the shaft member is press-fit into the bottomed bore, the air in the bottomed bore is permitted to be relieved from there through the opening, so that the air is prevented from being compressed within the bottomed bore and from applying any pressing force to the shaft-member in a direction in which the shaft-member is drawn out of the bottomed bore.
In such a conventional resin-product, to prevent the formation of the weld mark on the inner circumferential surface of the shaft bore formed extending along the center axis of the resin-product and to form the hole in the bottom portion of the shaft bore, it is necessary to take the following measure. As shown in FIG. 13, the pin gate 37 is provided on the top end 32a of the cavity 32 on the extension line of the center axis of the center pin 33. The molten resin 35 is injected from the pin gate 37 into the cavity 32 and solidified. Thereafter, the pin gate 37 filled with resin is removed and thus, the molding of a resin-product is completed. Subsequently, as described in the above-mentioned patent document, an opening is produced in the bottom of the shaft bore of the resin-product by post-working. In this case, the opening production by the post-working step is added resulting in an increase in the number of product manufacturing process steps, and an increase in the manufacturing cost for the resin-product must be invited.